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  • Chemical and structural changes in a pH-responsive mixed polyelectrolyte brush studied by infrared ellipsometry.

Chemical and structural changes in a pH-responsive mixed polyelectrolyte brush studied by infrared ellipsometry.

Langmuir : the ACS journal of surfaces and colloids (2009-07-04)
Karsten Hinrichs, Dennis Aulich, Leonid Ionov, Norbert Esser, Klaus-Jochen Eichhorn, Mikhail Motornov, Manfred Stamm, Sergiy Minko
ABSTRACT

This work provides direct chemical and structural insight into pH-dependent changes of an ultrathin (d=12 nm) mixed polyelectrolyte brush. In-situ infrared spectroscopic ellipsometry was used for the first time to study the gradual pH-responsive behavior of the brush, constituted of weak anionic and cationic polyelectrolytes, poly(acrylic acid) (PAA) and poly(2-vinylpyridine) (P2VP), respectively. The pH-dependent infrared fingerprints in the mid-infrared spectral range were analyzed as a function of chemical and structural changes in the mixed brush caused by pH changes. Thereby, the IR spectra were directly correlated to different chemical states of the brush, giving previously not accessible new information on the ionization of the thin film. In contrast to other techniques (e.g., classical attenuated total reflection IR spectroscopy) we used almost plane Si-substrates for the IR ellipsometric approach with application of a single reflection mode. The optical path through Si is of minimal length, which makes a large spectral range accessible. For the most pronounced bands of the carboxyl group at 1718 cm(-1) and the carboxylate ion at 1565 cm(-1), the band amplitudes were correlated with the degree of ionization of the carboxylic groups. Interpretation of the pH-dependent changes in the spectral signature reveals gradual changes of the chemical structures of the mixed brush between three distinct switchable states: strongly ionized PAA at pH 10, strongly ionized P2VP at pH 2, and mainly nonionized functional groups in a "dry" PAA-P2VP polyelectrolyte complex in the range from pH=4 to pH=7. At intermediate pH, the IR spectra confirm the previously made hypothesis of the formation of a polyelectrolyte complex between P2VP and PAA in the mixed brush. From IR spectra it is also concluded that the polyelectrolyte complex is formed as a result of a small fraction of ionized functional groups.

MATERIALS
Product Number
Brand
Product Description

Supelco
Poly(2-vinylpyridine), analytical standard, average Mw 159,000 (Typical), average Mn 152,000 (Typical)
Supelco
Poly(2-vinylpyridine), analytical standard, average Mw 37,500 (Typical), average Mn 35,000 (Typical)